The present invention relates to spin coating and more particularly to spin coating of chemicals in a controllable environment.
Spin coating is a known process for forming a layer of dispensed material on a rotating surface using the centrifugal force on the dispensed material. Typically, a substrate or a semiconductor wafer is held in chuck with a surface to be coated in a horizontal orientation. The chuck then spins causing the surface to be coated to rotate at a predetermined speed, and a dispenser then dispenses a predetermined amount of coating material in liquid form close to center of the rotation.
The rotation imposes a centrifugal force on the coating material which forces the mass of coating material to be pushed outwards away from the center of rotation towards the edges of the surface to be coated. As the coating material moves from the center to the edges, some of the coating material adheres to the surface and some of the coating material continues to flow outwards. In this way, a layer of coating material is formed over the surface to be coated. Excess coating material is either thrown off the edges or form a bead along the edges.
Spin coating is employed in a variety of applications such as forming photomasks and micromachining. Forming photomasks, for example, comprises the step of forming a layer of photoresist on a semiconductor wafer as part of a photolitographic process that forms circuit elements and components on the wafer. Spin coating is also used to form a layer of material on glass, forming a layer of dielectric material, and forming a layer of low-K material.
It is desirous that the layer of coating material formed by spin coating have a uniform thickness over the surface on which the coating material is applied. In addition, as coating material can be costly, the spin process should use only sufficient quantities of coating material as is necessary to form a layer with minimal wastage.
Presently, a spin coating apparatus comprises a two piece housing, the upper and lower pieces coming together to form an internal chamber. In the lower piece a chuck holds a work piece securely. For example, the chuck holds a semiconductor wafer. A quantity of coating material is then disposed at the center portion of the wafer, and the upper and lower housing pieces brought together enclosing the wafer and the coating material in the internal chamber. The chuck then rotates the wafer at a predetermined speed until the coating material has formed a uniform layer of coating material on the wafer.
The coating material contains solvent which begins to evaporate as soon as the coating material is dispensed. Using the conventional spin coating apparatus described above, during the time between dispensing the coating material and bringing the upper and lower housing pieces together, considerable quantities of solvent can evaporate. The lower the solvent content of the coating material, the lower the viscosity of the coating material. And, the lower the viscosity of the coating material the more difficult it is to spread the coating material evenly. Thus, resulting in a poorly formed layer of coating material, where the thickness of the coating material is not uniform. Non-uniform layers of coating material are not acceptable in a variety of subsequent processes. For example, when a layer of photoresist is not uniformly formed, the subsequent photolitographic processes result in inaccurately formed circuit elements and components.
Hence, one disadvantage of the known spin coating process is the time that lapses between dispensing coating material before the rotation is started. Another disadvantage is the lack of control of evaporation of the solvent from the coating material.
The present invention seeks to provide a method and apparatus for spin coating which overcomes, or at least reduces the above-mentioned problems of the prior art.
Accordingly, in one aspect, the present invention provides an apparatus for spin coating, the apparatus comprising:
a cover plate assembly comprising:
a rotatable cover plate having a first cavity, and the rotatable cover plate having at least one vent gaseously coupled to the first cavity; and
a base plate assembly for removably coupling to the cover plate assembly, the base plate assembly comprising:
a rotatable base plate having a second cavity, wherein the rotatable base plate includes a mount for securing a work piece in the second cavity,
wherein when the cover plate assembly and the base plate assembly come together, the rotatable cover plate couples to the rotatable base plate and the first cavity and the second cavity form a rotatable chamber with the work piece enclosed therein.
In another aspect, the present invention provides an apparatus for spin coating, the apparatus comprising:
a cover plate assembly comprising:
a rotatable cover plate having a first cavity and an opening gaseously coupled to the first cavity;
a manifold having an inlet coupled to the opening of the rotatable cover plate and having an outlet, wherein the outlet of the manifold is gaseously coupled to the first cavity through the opening, and wherein the manifold comprises a fluid controller for varying the degree of gaseous coupling between the first cavity and the outlet of the manifold; and
a coating material dispenser extending into the first cavity; and
a base plate assembly for removably coupling to the cover plate assembly, the base plate assembly comprising:
a rotatable base plate having a second cavity, wherein the rotatable base plate includes a mount for securing a work piece in the second cavity,
wherein when the cover plate assembly and the base plate assembly come together, the rotatable cover plate couples to the rotatable base plate and the first cavity and the second cavity form a rotatable chamber with the work piece enclosed therein.
In yet another aspect, the present invention provides a method for spin coating, the method comprising the steps of:
a) providing:
a cover plate assembly comprising:
a rotatable cover plate having a first cavity, and the first cavity having at least one vent gaseously coupled to the first cavity; and
a base plate assembly for removably coupling to the cover plate assembly, the base plate assembly comprising:
a rotatable base plate having a second cavity, wherein the rotatable base plate includes a mount for securing a work piece in the second cavity,
a work piece having at least one surface for spin coating; and
coating material;
b) determining speed of rotation, duration of rotation and quantity of the coating material to dispense from composition of the coating material, size of the vent, and desired thickness of the coating material to be formed on the at least one surface of the work piece;
c) mounting the work piece in the second cavity of the rotatable base plate;
d) securing the cover plate assembly and the base plate assembly together such that the rotatable cover plate couples to the rotatable base plate and the first cavity and the second cavity form a rotatable chamber with the work piece enclosed therein;
e) dispensing at least some of the quantity of coating material determined in step (b) on the at least one surface; and
f) rotating the cover plate assembly and the base plate assembly at the speed of rotation for the duration of rotation determined in step (b), thereby spreading the at least some of the quantity of coating material on the at least one surface to form a layer of coating material having the desired thickness.